402. Remove K Digits

Description

Given string num representing a non-negative integer num, and an integer k, return the smallest possible integer after removing k digits from num.

 

Example 1:

Input: num = "1432219", k = 3
Output: "1219"
Explanation: Remove the three digits 4, 3, and 2 to form the new number 1219 which is the smallest.

Example 2:

Input: num = "10200", k = 1
Output: "200"
Explanation: Remove the leading 1 and the number is 200. Note that the output must not contain leading zeroes.

Example 3:

Input: num = "10", k = 2
Output: "0"
Explanation: Remove all the digits from the number and it is left with nothing which is 0.

 

Constraints:

• 1 <= k <= num.length <= 105
• num consists of only digits.
• num does not have any leading zeros except for the zero itself.

 

Solutions

Solution: Stack

• Time complexity: O(n)
• Space complexity: O(n)

 

JavaScript

/**
 * @param {string} num
 * @param {number} k
 * @return {string}
 */
const removeKdigits = function (num, k) {
  const stack = [];

  for (const digit of num) {
    while (k && stack.length && stack.at(-1) > digit) {
      stack.pop();
      k -= 1;
    }
    if (!stack.length && digit === '0') continue;
    stack.push(digit);
  }
  return stack.slice(0, stack.length - k).join('') || '0';
};